Hi Mari,
Welcome to the forum.
I've just had a look around the immigration website and I see what you mean.
This website
Temporary Business (Long Stay) - Standard Business Sponsorship (Subclass 457) says that you must supply evidence of defacto relationship.
Have you searched through the posts on this forum? I have a feeling that this has come up before although I'm not sure if it was for that particular visa.
Regards,
Karen